The invention relates to a display device, in particular in a motor vehicle, with a liquid-crystal cell which has a pole filter, a front supporting panel, facing a viewer, a rear supporting panel and a liquid-crystal substance arranged in a cell space between the supporting panels, and with a heating device for heating the liquid-crystal cell.
Display devices of the above type are frequently used, for example, in motor vehicles for different indications and are therefore generally known. At low temperatures, the liquid-crystal cell of the display device is undesirably slow to respond on account of the then high inertia of the liquid-crystal substance. As a result, it takes a long time before a change of the indication can be read from the display. Since, in winter, initially very low temperatures may prevail in motor vehicles after starting, it is necessary to heat the liquid-crystal cell. This currently takes place by a heating device having heating wires, the heating wires being attached on a pole filter of the liquid-crystal cell. However, pole filters assigned to the liquid-crystal cell are heat-sensitive, so that their service life is reduced greatly when they are exposed to higher temperatures by the heating device. Therefore, there is a conflict of aims. On the one hand, it is desired for a cold liquid-crystal cell to be heated up as quickly as possible, and a relatively great heating output is required for this purpose, on-the other hand the pole filters must not be exposed to high temperature loading.
The invention is therefore based on the problem of providing a liquid-crystal cell of the type stated at the beginning in such a way that it can be heated up as quickly as possible without inadmissibly high temperature loading of the pole filter occurring.
This problem is solved according to the invention by the heating device being arranged outside the cell space and between the pole filter and one of the supporting panels, and by the pole filter being separated from the heating device by means of a thermal insulating layer.
Such an insulating layer allows a significantly higher heating output of the heating device than in the case of heating devices which, in the previously known form, have direct contact with the pole filter, without the pole filter being exposed to undesirably strong thermal stress. As a result, rapid heating up of the liquid-crystal cell is possible, so that even after cold nights a rapid indication on a liquid-crystal display becomes possible. Thanks to the invention, the service life of the pole filter, and consequently of the display device, is increased significantly with respect to comparable liquid-crystal cells. This is of great significance in particular in the case of display devices installed in motor vehicles. Such display devices, which are for example a component part of an indicating instrument cluster orxe2x80x94fitted into the dashboard or the central consolexe2x80x94of an information system, should not fail before the vehicle as a whole comes to the end of its service life, since they are very complicated to replace.
The utilization of the light emitted by an illuminating device for the liquid-crystal cell of the display device is particularly high if, according to an advantageous development of the invention, the insulating layer is transparent.
A fluid or a rigid body is conceivable for example as the insulating layer. The display device can, however, be produced at particularly low cost, and allows very effective thermal insulation of the pole filter with respect to the heating device, if the insulating layer is formed by a layer of air.
The heat-insulating layer of air or a space for the heat-insulating fluid is obtained in a particularly simple way if, according to another development of the invention, the pole filter is preferably mounted with a lateral frame on the heating device.
Alternatively, however, it is also possible for the insulating layer to be formed by a heat-insulating panel. On account of low production costs and simple handling, a glass or plastic panel is preferably appropriate here.
The heating device can be produced with the insulating layer as one component, and can consequently also be easily handled, if it is formed by an electrically conducting layer which faces the supporting panel, has an electrical resistance and on which the heat-insulating panel forming the insulating layer is formed. In the case of such an embodiment, liquid-crystal cells can without any problem be fitted with or without a heating device, which can be inserted as required. The heating of the liquid-crystal cell takes place in this case by the waste heat of the electrical resistance.
According to another development of the display device according to the invention, it is of particular advantage if the layer formed on the panel has a conductive polymer. As a result, the heating device can be produced in the particularly simple and low-cost way by means of the conductive plastic. The conductive polymer in this case makes high efficiency possible, as a result of uniform heat dissipation, without at the same time disturbing with any lasting effect the path of rays of the light passing through the liquid-crystal cell.
According to another advantageous development of the invention, just a small reduction in the light intensity, allowing a customary light source with low energy consumption to be used for illuminating the liquid-crystal cell, takes place if the conductive polymer has a light transmission of at least 90%.
If liquid-crystal cells are illuminated by means of transmitted light, a front pole filter and a rear pole filter are required. In such cases, it is advisable for both supporting panels each to have a pole filter, with the insulating layer interposed, for thermal isolation.
According to another advantageous development of the display device according to the invention, the latter has a DSTN (Double Super Twisted Nematic) cell, which has the liquid-crystal cell and a further liquid-crystal cell optically arranged in series with the liquid-crystal cell. In this case, the further liquid-crystal cell may be provided merely for color compensation as a passive cell, i.e. a cell which cannot be actively energized, or else be an active liquid-crystal cell which can be energized, for displaying information. With a DSTN cell, the display contrast can be improved considerably.
If the further liquid-crystal cell is to serve merely as a passive cell for color compensation and consequently for good, high-contrast readability even from great viewing angles, the display device according to the invention is advantageously further simplified if it has an FSTN (Foil Compensated Super Twisted Nematic) cell, which has the liquid-crystal cell and a color-compensating foil arranged optically in series with the liquid-crystal cell. This color-aberration-correcting compensating foil (also referred to as compensating film) may be arranged directly on the liquid-crystal cell orxe2x80x94without impairing its optical effectxe2x80x94at a distance from the liquid-crystal cell. This is advantageous in particular whenever the color-compensating foil is connected to the pole filter, forming a single component, consequently representing a pole filter with extended, color-compensating properties, and the heating device is in this way also to be arranged between this pole-filter color-compensating foil and one of the supporting panels of the liquid-crystal cell.